1899.1 OSTEOLOGY OF THE PYGOEOJJES. 1023 



The orhUosplunoid in the Divers is completely ossified ; in the 

 Grebes this is largely represented by membrane, so that the 

 anterior wall of the brain-case in the ch'icd skull bears a more or 

 less considerable fenestra. In a skull of yEchmopliorus kindly lent 

 me by Mr, Beddard this fenestra is very small, the orbitosphenoid 

 is ossified dorsad so as to close in and form tubes for the olfactory 

 nerves as they leave the brain. 



In the Divers the inlerorbital septum forms a vertical bar in 

 front of the optic foramen, this is wanting in the Grebes. 



The ethmoidal region. — The mesethmoid is indistinguishably 

 fused with the prespheuoid ( = interorbital septum) behind, and the 

 parasphenoidal rostrum below; it expands clorsally as usual into a 

 pair of lateral aliethmoidal plates under the nasal and frontal 

 bones, the free edges of which curve shghily downwards under 

 the outer border of the frontal and along the inner border of the 

 laclirymal. Its postero-dorsal border is continued backwards to 

 terminate in a sharp, spinous, crista-galli forming a median 

 partition between the olfactory nerves. 



The antorbital plate in the Divers is represented by a thin ridge 

 of bone running from the mesethmoid outward and forward to 

 the lachrymal. In the Grebes this is represented by a narrow 

 band-shaped scroll of bone from the lower and hiuder border 

 upwards to the nasal, immediately to the inner side of the dorsal 

 end of the lachrymal. 



A comparison may profitably be made here between the mes- 

 ethmoid of the Pygopodes and that of the Impennes and Tubinares. 

 In the two first mentioned groups the mesethmoid is relatively 

 smaller than in the last, and only very slightly pneumatic. 



In the Divers and Grebes its anterior border curves gently 

 forwards, carrying with it a pair of lateral wing-like ridges, the 

 whole eventually terminatiugin a sharply truncated border running 

 transversely across the skull immediately under the free end of the 

 nasal processes of the prem axilla. Its posterior border is deeply 

 hollowed by the interorbital fenestra. Its dorsal border, anteriorly, 

 expands into a pair of lateral aliethmoidal plates, tapering from 

 before backwards ; posteriorly it runs backwards, in the Divers in 

 the form of a deep, and in the Grebes in the form of a very narrow 

 knife-like ridge, the free end of which terminates as a pointed 

 " crista-galli " within the olfactory fossa. 



In the Impennes the form and relations of the mesethmoid 

 closely resemble those of the Pygopodes. 



In the Tubinares, the mesethmoid differs from the forms de-- 

 scribed on account of the fact that its upper and lower regions are 

 brought into sharp contrast by reason of the great pneumaticity 

 of the lower region, which causes the upper non-pneumatic half, 

 with its gently arched aliethmoidal wings, to assume a cavern-like 

 form, which passes backwards in a tubular manner into the 

 olfactory fossa. Moreover, the crista-galli takes the form of a 

 median " pillar dividing two large tubular apertures for the 

 olfactory crura ; whilst in the other forms the crista-galli is 



66* 



